The bonding of thermoplastic material through the application of heat and pressure is well known in the prior art. Devices to accomplish such bonding have commonly taken the form of a supporting platen and heated die combination. The materials to be bonded are placed upon the platen and the heated die is pressed on the material. It is critical in this type of device that the surface of the die be precisely controlled. That is, any skewing of the die surface from the plane of the supporting platen results in an excessive extrusion of the bonded materials and an unacceptable bond. Indeed, even with the proper orientation of the die surface, the uneven pressure applied to the surface of the materials or, more precisely, the application of pressure only in the site of the intended bond unfailingly produces an extrusuion of the materials thereby causing a severe weakening of the bond. Further, variations in material thickness causes an effect similar to that caused by a skewed die surface which cannot be compensated for in this device.
An alternative method of bonding thermoplastic materials is disclosed in U.S. Pat. No. 2,705,523. In this patent, there is disclosed a device having a working surface upon which the materials to be bonded are placed. A hood is lowered into contact with the materials to form a pressure chamber in combination with the materials. The chamber is then pressurized to a pressure typically higher than 50 psi which pressure becomes the necessary bonding pressure. This system differs from the die bonding system in that the bonding pressure is applied uniformly over the surface of the materials and not just in the area which is to be bonded together.
Although the system of U.S. Pat. No. 2,705,523 can provide a bond superior to that obtainable in the die bonding system, it nevertheless poses significant operational disadvantages. First, an effective seal must be accomplished between the hood and the materials to be bonded. This requires that the hood be precisely aligned with respect to the working surface and the materials. In addition, this system makes no provision for the removal of air which may be trapped between the various layers of the materials. Also, the system does not offer the possibility of accelerating the cooling of the bonded area while the disclosure states that the pressure on the materials being bonded is maintained until such time as the materials have cooled. By accelerating the cooling of the bonded area, it would be possible to move the bonded materials, without damage to the bond, much faster than is possible in a device which relies upon natural cooling. This inability to accelerate cooling of the bonded area necessarily places a limitation on production rate, thereby limiting the utility of the system itself. Finally, the system of U.S. Pat. No. 2,705,523, results in a gradual but certain increase in the temperature of the working surface thus limiting the ability to control the sealing temperature range of the materials being sealed.